userspace: add vxlan gpe support to vport

userspace: add vxlan gpe support to vport

This patch is based on the "datapath: enable vxlangpe creation in compat mode"
from Yi Yang. It introduces an extension option "gpe" to the vxlan port in the
netdev-dpdk datapath. Description of vxlan gpe protocoll was added to header
file lib/packets.h. In the vxlan specific methods the different packet are
introduced and handled.

Added VXLAN GPE tunnel push test.

Signed-off-by: Yi Yang <yi.y.yang at intel.com>
Signed-off-by: Georg Schmuecking <georg.schmuecking@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>